The lung of adult mammals is the target for a wide range of environmental toxicants. Many of these toxicants including furans, and chlorinated and aromatic hydrocarbons, require metabolism by the cytochrome P450 monooxygenase system to produce toxicity. The nonciliated bronchiolar epithelial (or Clara) cell is the primary target cell for a large number of these compounds. Clara cells differentiate postnatally in most species. Differentiation includes both the acquisition of cellular features characteristic of mature Clara cells and a 1O-fold increase in pulmonary cytochrome P450 monooxygenase activity during the postnatal period. Whether these differences in cellular characteristics and P450 monooxygenase expression alter Clara cell susceptibility or affect lung toxicity in neonates and postnatal animals is not known. A preliminary study of 4-ipomeanol-induced Clara cell cytotoxicity indicates that immature Clara cells of one-week-old rabbits are much more susceptible to injury than are Clara cells in adults. The overall purpose of this proposal is to test the hypothesis that differences in the susceptibility of mature and differentiating Clara cells to P450-activated lung cytotoxicants result from one or more of three intrinsic cellular metabolic characteristics: 1) differences in cytochrome P450 monooxygenase activity; 2) differences in activity of detoxifying enzyme systems (glutathione S-transferases, epoxide hydrolases and glucuronyl transferases); 3) differences in glutathione metabolism and regulation of the cellular glutathione pool. We propose to take a two-stage approach to addressing this hypothesis. First, we will define the pattern of acute toxicity in differentiating Clara cells. We will use two model compounds, an aromatic hydrocarbon (naphthalene) and a furan (4-ipomeanol) to compare Clara cell cytotoxicity in neonatal animals with that in adults. We will compare two species, the mouse, which is injured by both naphthalene and 4-ipomeanol, and the rabbit, which is injured by 4-ipomeanol but not by naphthalene. We will compare the metabolism of these two cytotoxicants in preparations from whole lungs and microdissected airways of postnatal animals of both species. In adults, they are activated by two different isozymes. We will define: 1) the impact of P450-mediated Clara cell cytotoxicity in newborns on the pattern of postnatal Clara cell differentiation; 2) the pattern of expression of enzymes involved in detoxifying reactive intermediates during Clara cell development in the postnatal lung; and 3) cellular glutathione metabolism during postnatal Clara cell development. We propose to use a multidisciplinary approach combining developmental, pathobiological and biochemical studies. The long-range goal is to provide an understanding of the cellular and metabolic basis for the differences in susceptibility to pulmonary cytotoxicants between differentiating and mature Clara cells.